1. Technical Field
The present invention relates to a system in which a content, which is a digitalized literary work such as a movie or a musical piece, is recorded onto, and reproduced from, a large-capacity recording medium such as an optical disc, and particularly to a copyright protection system which prevents a content from being used illicitly, without permission from the copyright owner.
2. Background Art
In recent years, businesses which market contents, which are digitalized literary works such as movies and musical pieces, by storing such contents in optical discs, for example, are being carried out increasingly, following the increased capacity of recording media.
As there is a possibility that contents stored in a recording medium will be illicitly copied, some kind of protection becomes necessary.
In general, in order to protect the copyrights of contents, more specifically, to prevent illicit usage such as illicit reproduction and illicit copying of a content, encryption technology is being utilized.
To be more specific, a content is encrypted using a certain encryption key, recorded on a recording medium such as an optical disc, and distributed. At the same time, only a terminal which holds a decryption key corresponding to such encryption key can decrypt, using the decryption key, data which is read out from the recording medium, and carry out the content's reproduction, and so on.
Moreover, as methods for encrypting a content and recording the encrypted content in a recording medium, there exists (a) a method for encrypting the content itself, using an encryption key which corresponds to a decryption key held by a terminal, and recording the encrypted content, and (b) a method in which, in addition to encrypting the content using a certain key and recording the encrypted content, a decryption key corresponding to such key is encrypted using an encryption key corresponding to a decryption key held by the terminal, and the encrypted key is recorded.
At this time, strict management is required so that the decryption key held by the terminal is not revealed to the outside. However, there is a danger that certain keys may be exposed to the outside through a cryptanalysis of the inside of the terminal by an unauthorized person. Once certain keys are exposed to an unauthorized person, it is conceivable that contents can be circulated through the Internet by creating a reproduction apparatus or software for illicitly using contents. In such a case, it is assumed that a copyright owner would want that a key which has been exposed once, would not be able to handle contents to be provided subsequently. Technology for realizing this is referred to as key revocation technology, and a system which realizes key revocation is disclosed in patent reference 1 (Japanese Laid-Open Patent Application 2002-281013 Publication).
On the other hand, as apparatuses which reproduce the encrypted content recorded on the recording medium, there exists (a) a so-called household player in which a function for reading-out the encrypted content from the recording medium, and a function for decrypting the encrypted content that was read out, are integrated, and (b) a reproduction apparatus which reads-out the content from the recording medium using an optical disk drive connected to or built into a personal computer, then decrypts the read-out encrypted content using an application program operating on the host of a personal computer, and reproduces the decrypted content. Non-patent reference 1 (Content Protection for Prerecorded Media DVD Book, 4C Entity, LLC) discloses copyright protection systems corresponding to these two types of reproduction apparatuses.
However; as revocation data, which is common for all types of the subject reproduction apparatuses, is recorded onto the recording medium in such conventional copyright protection systems as those mentioned above, each reproduction apparatus needs to be provided with a memory within the apparatus, for storing, at least temporarily, the entirety of such revocation data read out from the recording media.
Furthermore, generally, in a household player such as a DVD player, changing the process algorithm and key length built into the apparatus is difficult, taking time and effort.
On the other hand, compared to implementation using hardware, when a decryption process and key are implemented as an application program on the personal computer, through software, updating and additions to the internal encryption algorithm and key is easy, but sturdy implementation of the encryption algorithm and key is difficult. However, in the conventional copyright protection system in which the common revocation data is recorded on the recording medium, even when the application program operating on the host of the personal computer is illicitly cryptanalyzed, and the algorithm and a number of keys are exposed, changing the encryption/decryption algorithm and key lengths is virtually impossible. This means that the revocation function will not be able to operate properly, leading to the spread of the illicit use of the content using an illicit device. Furthermore, once the key or algorithm of the application used in a personal computer is exposed, it is possible to have a case where the revocation functions in all devices, including consumer devices, will stop operating properly.
In order to solve the aforementioned problem, the present invention provides a copyright protection system which (a) enables the size of a memory provided inside the reproduction apparatus to be reduced, and (b) enables the revocation function of the entire system to be maintained, even when the application program operating on the host of the personal computer is cryptanalyzed and the algorithm and a number of keys are exposed, by changing the encryption/decryption algorithm and key lengths.